1. Field of the Invention
The present invention relates to a fixing unit for fixing a transfer image on paper, which is included in, for example, an electronic copying machine, and to an image forming apparatus having the fixing unit.
2. Description of the Related Art
An electronic copying machine includes a photosensitive body, and a latent image is formed on the photosensitive body by an image forming means. The latent image is supplied with a developer and thus developed. The developed image is transferred to paper by the operation of a transfer charger, and the paper is fed to a fixing unit, thereby fixing the image on the paper. The fixing unit includes a fixing roller and a pressing roller pressed on the fixing roller. The paper is caused to pass between the fixing and pressing rollers, and heated and pressed accordingly. The pressing roller is attached to a housing of the fixing unit, which is freely opened and closed, and contacts/separates from the fixing roller by opening/closing the housing. Normally, the pressing roller remains pressed against the fixing roller, irrespective of ON and OFF states of a power supply.
If an electronic copying machine is left alone for a long time and then its power is turned on, irregularity in temperature may occur in the fixing unit even in the ready state. Particularly in an electronic copying machine in which paper is heated only by a heater of a fixing roller, it goes into the ready state even when a portion opposite to a portion of a pressing roller pressed against the fixing roller remains cold. If, therefore, paper is caused to pass between the fixing and pressing rollers, a difference in temperature between both sides of the paper is considerably widened, and the paper is greatly curled. Even though the paper is not greatly curled, a low-temperature portion of the paper absorbs heat and a defect in fixing occurs accordingly.
In order to prevent the paper from being curled or to prevent the defect in fixing, a conventional copying machine has a technique of rotating the fixing and pressing rollers to uniform the surface temperature of the pressing roller when the copying machine is warmed up.
However, the warm-up time of an electronic copying machine is considered to be as important as the quality of an image, in view of its usage. If the pressing roller is rotated, it takes 50% longer time to warm up the machine than when the pressing roller is not rotated. Needless to say, this is because the pressing roller always contacts the fixing roller and the fixing roller absorbs heat as the pressing roller rotates.
A pressing roller of low hardness has recently been used to increase the rate of fixing. If, however, such a pressing roller is pressed against a fixing roller, a great creep occurs, resulting in a blur of fixing. Further, in a preheat mode, for keeping the temperature of the fixing roller lower than the usual temperature for fixing since the pressing roller remains pressed against the fixing roller, the pressing roller cannot be heated uniformly in the former case.